Identification of Fungi based on microscopic feature (Yeasts and Molds)

Tankeshwar Acharya

4 years ago

If you observe a probable fungus in the culture plate, colony characteristics are first assessed to determine the broad group of the isolate. Once the initial observations are made, following microscopic criteria are used to make a genus/species identification of the fungal isolate.

If hyphae are observed, determine the structure of the hyphae. Are they:

Septate or aseptate

Branching (if so, at what angles) or not branching?

Pigmented or not pigemented

Even or uneven in width

Composed of arthroconidia or pseudohyphae

Determine the structure and derivation of fruiting bodies

Visualize the type of conidation:

The size and shape of the spores or conidia

The size, shape, and arrangement of the spores

Look for the presence of special diagnostic structures: pycnidia, cleistothecia, Hulle cells

If only yeast cells are observed:

Note their size, shape and arrangement.

Look for the presence of or absence of capsules.

Determine the type of bastoconidiation- are daughter cells single or multiple?

Presumptive identification of fungi based on Direct Microscopic Examination of material from Clinical Specimen.

Yeast forms, cells spherical and irregular in size (5-20 micrometer), classically with a thick polysaccharide capsule (not all cells are encapsulated), with one or more buds attached by a narrow constriction: Cryptococcus neoformans, Cryptococcus spp., nonencapsulated

Small budding yeast, relatively uniform in size (3-5 micrometer), with a single bud attached by a narrow base, extracellular or within macrophages: Histoplasma capsulatum

Yeast forms, large (8-20 micrometer), with cells appearing to have a thick, double-contoured wall, with a single bud attached by a broad base: Blastomyces dermatitidis